If you like him, ask him out...if you're not interested, politely tell him (in private- don't want to embarassd the poor fellow!) that you simply see him as a friend and nothing more but you really like him as a friend and value that friendship. Tell him, you hope you can continue to be friends and say you're there for him if he ever needs to talk. :)
This is where experience comes in, holld his hand, put your arm through his, walk closer to him than normal, when your talking face to face, straighten his tie, if he doesn't wear one, pretend his collar needs smoothing, and do it in a suggestive manner, looking into his eyes.
Do you want to approach him to say that you like him too or to say it's not going to happen?
If you like him and you know he likes you then I recommend knocking on his door and when he answers just kissing him, smiling and walking away without a word... sounds barking but trust me, it's worth it.
If you want to tell him you don't like him then consider whether it's worth doing this or whether you can get away with just glossing over it. If it really needs addressing then do it in private and gently.